1. Field of the Invention
The present invention relates to a motor vehicle steering angle detector for detecting the steering angle of a steering mechanism of a motor vehicle. The invention further relates to a power steering system employing such a steering angle detector, more particularly, to a power steering system which applies a steering assist force to a steering mechanism by a hydraulic pressure generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering systems are conventionally utilized which assist the operation of a steering wheel by supplying a working oil from an oil pump into a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor.
When the steering is not turned, no steering assist force is required. Therefore, a so-called xe2x80x9cstop-and-go controlxe2x80x9d is performed, wherein the electric motor is deactuated when the steering assumes a straight travel steering state virtually at a steering angle midpoint and, in response to detection of a steering angle change greater than a predetermined level, the electric motor is actuated.
Detection of the steering angle midpoint is based, for example, on an output of a steering angle sensor provided in association of the steering mechanism. The steering angle sensor is adapted to detect a steering angle change with respect to an initial steering angle, i.e., a relative steering angle. The initial steering angle is a steering angle observed when an ignition key switch is turned on. Accordingly, the initial steering angle does not always coincide with the steering angle midpoint, but often corresponds to a steering angle observed when the steering wheel is turned. Therefore, steering angle data for the steering angle midpoint is determined, for example, by sequentially sampling steering angle data for determination of frequency of occurrence of steering angle data, and employing the most frequent steering angle data as the steering angle midpoint data.
The sampling of the steering angle data (relative steering angle data) for the determination of the steering angle midpoint is carried out, for example, on condition that a steering angular speed is virtually zero or the steering wheel is held still (with no steering operation) for at least a predetermined period.
However, the detection of the steering angle midpoint requires a relatively long period of time after the turn-on of the ignition key switch. During this period, a control operation for deactuation of the electric motor cannot be performed, making the energy saving of the power steering system difficult.
It is an object of the present invention to provide a motor vehicle steering angle detector which is capable of detecting a steering angle immediately after an ignition key is turned on, thereby contributing, for example, to improvement of energy saving of a power steering system.
It is another object of the present invention to provide a power steering system which is capable of starting a control operation for bringing an electric motor into an energy saving mode at a relatively early time point after an ignition key is turned on for improvement of energy saving thereof.
In accordance with the present invention, there is provided a steering angle detector for a motor vehicle, which comprises: a steering angle detecting section which receives an initial value applied thereto when an ignition key of the motor vehicle is turned on, and detects a relative steering angle indicative of a direction and degree of a steering operation; a steering angle midpoint computing section which repeatedly performs a computation during traveling of the motor vehicle to determine a steering angle midpoint value which corresponds to an output value of the steering angle detecting section indicative of a steering angle midpoint; an absolute steering angle calculating section which calculates an absolute steering angle on the basis of the steering angle midpoint value determined by the steering angle midpoint computing section and the relative steering angle detected by the steering angle detecting section; a rewritable nonvolatile memory for storing therein a latest steering angle midpoint value determined by the steering angle midpoint computing section; and a section for setting the steering angle midpoint value stored in the nonvolatile memory as a tentative steering angle midpoint value for the calculation of the absolute steering angle before the steering angle midpoint is newly determined by the steering angle midpoint computing section immediately after the turn-on of the ignition key. The detector preferably further comprises a validity judging section for judging validity of the tentative steering angle midpoint value.
According to the present invention, the steering angle midpoint value determined by the steering angle midpoint computing section is stored in the rewritable nonvolatile memory, and retained therein even after the ignition key is turned off. Since the steering angle midpoint value determined in the previous traveling of the motor vehicle is retained in the nonvolatile memory, the absolute steering angle calculating section performs the absolute steering angle calculation process by employing the steering angle midpoint value as the tentative steering angle midpoint value immediately after the ignition key is next turned on.
When a steering operation is performed during an off-period of the ignition key, however, a relationship between an actual steering angle of a steering mechanism and the steering angle midpoint value stored in the nonvolatile memory is changed from that observed immediately before the turn-off of the ignition key. This means that the tentative steering angle midpoint value read out of the nonvolatile memory is not always valid. Therefore, it is preferred to provide the validity judging section for judging the validity of the tentative steering angle midpoint value.
The steering angle midpoint computing section determines the steering angle midpoint value by sampling outputs of the steering angle detecting section each indicative of a relative steering angle, counting the outputs to determine the frequency of occurrence of outputs for each output value for preparation of a histogram, and defining the most frequent output value as the steering angle midpoint value. The judgment of the validity of the tentative steering angle midpoint value is more easily achieved than the determination of the steering angle midpoint value. Therefore, the process to be performed on condition that a valid steering angle midpoint value is determined or on condition that a correct absolute steering angle is determined can be started at a relatively early time point after the turn-on of the ignition key of the motor vehicle.
The validity judging section may be adapted to judge that the tentative steering angle midpoint value is valid if the motor vehicle travels a distance not smaller than a first predetermined distance with the absolute steering angle being kept within a predetermined steering angle range including the tentative steering angle midpoint value. In this case, the validity judging section maybe adapted to judge that the tentative steering angle midpoint value is invalid unless the tentative steering angle midpoint value is judged to be valid before a traveling distance of the motor vehicle after the turn-on of the ignition key reaches a second predetermined distance greater than the first predetermined distance.
With this arrangement, the tentative steering angle midpoint value is judged to be valid if the motor vehicle travels a distance not smaller than the first predetermined distance with the absolute steering angle being kept within the predetermined steering angle range including the tentative steering angle midpoint value. That is, if the relationship between the tentative steering angle midpoint value and the actual steering angle of the steering mechanism is not significantly changed from that observed immediately before the previous turn-off of the ignition key, the absolute steering angle calculated by the absolute steering angle calculating section on the basis of the tentative steering angle midpoint value is kept within the predetermined steering angle range including the tentative steering angle midpoint value when the motor vehicle travels straight. If the steering operation is performed during the ignition key off-period, the relationship between the tentative steering angle midpoint value and the actual steering angle of the steering mechanism is significantly changed. Accordingly, the absolute steering angle calculated by the absolute steering angle calculating section is not kept within the predetermined steering angle range including the tentative steering angle midpoint value even if the motor vehicle travels straight. Therefore, the tentative steering angle midpoint value is judged to be invalid unless the tentative steering angle midpoint value is judged to be valid before the traveling distance of the motor vehicle after the turn-on of the ignition key reaches the relatively great second predetermined distance.
Further, if the steering angle midpoint value is newly determined by the steering angle midpoint computing section after the turn-on of the ignition key, the steering angle midpoint value thus determined is preferably employed instead of the tentative steering angle midpoint value.
With this arrangement, upon the determination of the steering angle midpoint value by the steering angle midpoint computing section, the steering angle midpoint value thus determined is employed instead of the tentative steering angle midpoint value, and stored in the nonvolatile memory. This thereafter makes it possible to correctly determine the absolute steering angle.
The detector preferably further comprises a rewritable nonvolatile relative-steering-angle memory for storing therein a relative steering angle value detected by the steering angle detecting section immediately before the turn-off of the ignition key, and an initial relative steering angle setting section for setting the relative steering angle value stored in the rewritable nonvolatile relative-steering-angle memory as an initial output value of the steering angle detecting section after the turn-on of the ignition key.
With this arrangement, when the ignition key is turned on, the relative steering angle value detected at the previous turn-off of the ignition key is set as the initial output value of the steering angle detecting section. Therefore, a relationship between the output value of the steering angle detecting section and the tentative steering angle midpoint value observed immediately before the turn-off of the ignition key can be maintained. Thus, the absolute steering angle can correctly be calculated immediately after the turn-on of the ignition key, as long as the tentative steering angle midpoint is valid.
The storage of the relative steering angle in the rewritable nonvolatile relative-steering-angle memory immediately before the turn-off of the ignition key is achieved, for example, by storing an output of the steering angle detecting section in the rewritable nonvolatile relative-steering-angle memory when the speed of the motor vehicle detected by a vehicle speed sensor is zero.
The rewritable nonvolatile relative-steering-angle memory and the nonvolatile steering-angle-midpoint memory may be provided in different storage areas in the same storage device.
In accordance with the present invention, there is further provided an electric power steering system for generating a steering assist force to be applied to a steering mechanism by utilizing a hydraulic pressure generated by a pump driven by an electric motor, the electric power steering system comprising: a motor vehicle steering angle detector having the aforesaid features; and an energy saving mode transition controlling sections for bringing the electric motor into an energy saving mode essentially on condition that an absolute steering angle detected by the motor vehicle steering angle detector is kept within a predetermined steering angle range including a steering angle midpoint value for a predetermined time period.
According to the present invention, the essential condition for bringing the electric motor into the energy saving mode is that the steering angle of the steering mechanism is kept within the predetermined steering angle range including the steering angle midpoint value for the predetermined time period. When the steering angle midpoint value is not sure immediately after the turn-on of the ignition key, therefore, the control operation for bringing the electric motor into the energy saving mode can speedily be started by employing a tentative steering angle midpoint value. This improves the energy saving of the power steering system.
The term xe2x80x9cenergy saving modexe2x80x9d herein means a mode in which the electric motor is operated with a lower power consumption, for example, a mode in which the electric motor is deactuated or a mode in which the electric motor is driven at an idling rotation speed which is lower than a minimum rotation speed (stand-by rotation speed) thereof employed for a steering assist operation.